The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the present invention. Some such contributions of the present invention may be specifically pointed out below, while other such contributions of the present invention will be apparent from their context.
A random access procedure is a fundamental procedure for a UE to have access to a wireless network, such as a long term evolution (LTE) network. The UE may perform a random access procedure upon occurrences of the following exemplary situations: when the UE performs an initial connection with the radio network node (e.g., base station, BS) if there is no connection (e.g., a radio resource control (RRC) connection); when the UE first accesses a target cell in a handover procedure; when requested by a command from the BS; when time synchronization of the uplink does not match, or the designated radio resources to be used for requesting radio resources have not been allocated, but data of the uplink is generated; and a recovery procedure when there is a radio link failure or handover failure.
In the LTE system, such random access procedures can be divided into a contention based random access procedure and a non-contention based random access procedure. Such division relies on whether the random access preamble used during the random access procedure is selected by the UE itself or selected by the BS.
In the non-contention based random access procedure, the UE uses the random access preamble that was directly allocated to it by the BS. Thus, when the BS allocates a particular random access preamble to the UE, only that UE uses such random access preamble and other UEs cannot use this random access preamble. Thus, because the random access preamble and the UE utilizing that random access preamble have a one-to-one relationship, there are no collisions (or contentions) in the random access procedure. In this case, once such random access preamble is received by the BS, the identity of a UE that transmitted the random access preamble can be known.
In contrast, in the contention based random access procedure, among a plurality of random access preambles that the UE can use, one is randomly chosen and transmitted, and thus there is likelihood that a plurality of UEs may use the same random access preamble. Thus, upon receipt of a particular random access preamble, the BS is not able to ascertain which UE transmitted such random access preamble.
For a better understanding of the random access procedure, exemplary descriptions will be made to the contention based random access procedure in connection with FIG. 1.
FIG. 1 illustrates a flow chart of the contention based random access procedure 100 between the UE and the BS. As illustrated in FIG. 1 and also as mentioned above, based on the indication of a group of random access preambles provided through system information or a handover command, the UE randomly selects one random access preamble and then selects physical random access channel (PRACH) resources that can be used to transmit such a random access preamble, and performs transmission of the random access preamble at step S101. Here, the preamble may be referred to as a random access channel (RACH) message (MSG) 1.
Subsequent to transmitting the random access preamble as above, the UE, at step S102, receives its random access response message (also referred to as “RACH MSG 2”) within a random access response reception window that was indicated by system information or handover command from the BS. Here, the random access response message includes values such as a preamble identifier (ID), a UL Grant (uplink radio resources), a temporary Radio Network Temporary Identifier (TC-RNTI), and a time alignment command, and the like. Here, if a random access preamble identifier is needed, and because random access response information intended for one or more UEs can be included in a single random access response, the UL grant, temporary C-RNTI, and Time Alignment Command information are used to inform of the UE for which such information is valid. The random access preamble identifier is the same as the random access preamble that was selected in the procedures mentioned above.
Upon receipt of the valid random access response at step S102, the UE processes the information included in the random access response respectively. For example, the UE applies the Time Alignment Command and stores the temporary C-RNTI. Additionally, by using the UL Grant, the data stored in the buffer of the UE or newly generated data are transmitted to the BS at step S103 via a scheduled transmission message, which is also referred to as a RACH MSG 3. Among the data included in the RACH MSG 3, the identifier of the UE should essentially be included such that the BS can determine which UEs performed the random access procedure and identifying each UE allows future collisions to be resolved. Generally, if the UE transmitted the scheduled transmission message through the UL Grant, a contention resolution timer would be started.
Following scheduled transmission of step S103, the UE waits for instructions from the BS for contention resolution (or to resolve any contention). Namely, an attempt to receive a contention resolution message (also referred to as MSG 4) over the PDCCH is made. There may be two ways to receive the contention resolution message over the PDCCH at step S104. The first way is to attempt PDCCH reception by using the cell identifier, if the cell identifier is its identifier that was transmitted via the UL Grant. The second way (i.e. if its identifier is a unique identifier) is to use the Temporary C-RNTI included in the random access response in attempting to receive the PDCCH.
In the above first way, if the PDCCH is received through its cell identifier before expiration of the contention resolution timer, the UE determines that the random access procedure was performed successfully. In the above second way, if the PDCCH was received through the temporary cell identifier before expiration of the contention resolution timer, the data within the Physical Downlink Shared Channel (PDSCH) is checked according to the indication of the PDCCH. The data within the PDSCH is referred to as RACH MSG 4. If the above data includes its unique identifier, the UE determines that the random access procedure was performed successfully.
The foregoing briefly introduces the random access procedure in which related four messages have been discussed. As to the MSG 4 therein, the eNB is likely to choose a conservative Modulation and Coding Scheme (MCS) and thus assign the fixed number of PRBs for transmission of the MSG 4. In general, this conservative MCS is unreasonable for transmission of the MSG 4 since it may occupy superfluous Physical Resource Blocks (PRBs) and thus the scare resources would be unnecessarily wasted. As a result, either the throughput of the UEs that are not in the random access procedures will be negatively affected or the random access delay of the UEs during the random access procedures will be lengthened.
In view of the foregoing, it would be desirable to provide solutions that enable highly efficient random access procedures such that PRBs for transmission of the MSG 4 can be reasonably assigned and potential waste of the PRB in this regard could be avoided.